BaseTools/GenFds: remove the old logic since ActivePlatform is abs. path

BaseTools/GenFds: remove the old logic since ActivePlatform is abs. path

We can support the DSC file out of workspace. this old logic first make
the absolute path to relative path and strips the leading slash off,
then append it to workspace. it cause GenFds failure on Linux when the
DSC file is out of workspace. Since we make sure the ActivePlatform is
abs. path, so we don't need this old logic to change the abs. path to
relative.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Marvin Haeuser <marvin.haeuser@outlook.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Yonghong Zhu <yonghong.zhu@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>